After multifilter photometry of the W UMa-type contact binary VW Cep (P = 6.67 hr; G5V + K0V) in 1986/87 revealed large asymmetries in the light curves believed to be caused by large, cool starspot regions on the surface of the larger stars, in Apr. 1987 it was observed with IUE to study the chromospheres and transition regions of the components. During one complete orbital cycle, three SWP and four LWP low dispersion spectra were obtained, including and then excluding the suspected active region. Phase dependent TR line emission strengths were found, most notably C IV, which is 50 percent stronger when the spot region is most visible. The results could be important because VW Cep represents an extreme case for studying stellar dynamo theory and observations can play a crucial role in the unterstanding of magnetic fields and activity cycles in rapidly rotating solar-like stars

Bradstreet, David H.

Guinan, Edward F.

English

NASA Technical Reports Server

- ,z. 303
N89-10142
MAPPING OF SURFACE ACTIVITY
ON THE W UMA-TYPE SYSTEM VW CEPHEI
David H. Bradstreet
Dept. of Physical Science
Eastern College
St. Davids, PA 19087
USA
Edward F Guinan
Dept. of Astronomy & Astrophysics
Villanova University
Villanova, PA 19085
USA
ABSTRACT
Multlfllter photometry of the W UMa-type contact binary VW Cep
(P = 6.67 hr; G5V + KOV) m 1986/87 revealed large asymmetries
in the light curves believed to be caused by large, cool starspot
regions on the surface of the larger star. In Apnl 1987 we
observed VW Cep with IUE to study the chromospheres and
transmon regions of the components. During one complete
orbital cycle 3 SWP and 4 LWP low dispersion spectra were
obtained including and then excluding the suspected active reglon.
For the first time, phase dependent TR hne emission strengths
were found, most notably C IV, which was 50% stronger when
the spot region was most visible. These results could be important
because VW Cep represents an extreme case for studying stellar
dynamo theory and observations can play a crucial role in the
understanding of magnetic fields and activity cycles in rapidly
rotating solar-like stars.
Keywords: Cool Stars, W UMa Binaries, Starspots,
Chromospheric Activity, Stellar Dynamos
1. INTRODUCTION
Observations of W UMa-type binaries with the IUE have shown
that these stars are rich sources of ultraviolet emission. They
generally have large surface fluxes of high temperature lines such
as C 1I, C IV, NV, and S IV as well as moderate temperature (104
K) chromospheric emission lines of Mg 11, Lyct, C 1,O 1,and Si
II. In analogy with the chromosphencally active RS CVn stars,
these hne emissions are thought to ar|se from dynamo generated
magnetic fields since these stars possess tidally enforced rapid
rotation and deep convective zones, at least for the shorter period
systems. However, because of their strong udal interactions,
components of W UMa systems should not possess significant
dlfferentml rotation (both radxal as well as latitudinal) which is
believed to be an important component of stellar dynamo theory
(Refs. 1,2).
The hght curves of most of the shorter period W UMa systems,
of G to K spectral type, display markedly asymmetric light
curves. Modeling of light curves using cool spotted regions can
account for these asymmetries (Ref. 3). The presence of
starspots on these stars is not unexpected because of their very
short orbital and rotational periods. However, the W UMa-type
stars are not as active as s_mple extrapolations of trends for
detached binaries (the RS CVn stars) would indicate (Ref. 4).
This could indicate a saturation of the dynamo mechanism in these
stars.
2. PIIOTOMETRY OF VW CEPIIEI
VW Cep (liD 197433) is one of the brightest (Vma x __ + 7.8 mag),
best observed short period W UMa-type binaries. It consists of ~
G5V and ~ K0V components in contact with their Roche hmlting
surfaces. Photoelectric photometry has been carried out on this star
since 1948. Since 1978, VW Cep has been monitored
photoclecmcally at Vfllanova University Observatory. These light
curves were combined with others obtained over the same time
interval and found to change systematically with time. Dunng
1979-81 asymmetries are evident in the light curves with the
brightness of maximum I (at 0.25P) and maximum II (at 0.75P) as
well as minimum I (at 0.0P) and minimum 1I (at 0.50P) changing
up to 0.07 mag (in yellow) with a characteristic penod of ~ 155 _+5
days. The light curves obtained during 1982-84 appear to be
disturbed the least amount. Beginnmg in 1985, and continuing
through 1987, the light curves are asymmetric with max I being ~
0.06 mag brighter than max 11. The long term behavior of the
system is illustrated in Fig. 1 in which the differences between the
brightness of the maxima at 0.25P and 0.75P for a given light
curve are plotted against time in the sense [mag(max 11)- mag(max
I)]. This gives a rough measure of the asymmetry present m the
light curves. A value of 0.0 for th_s quantity corresponds to a time
when the two maxima of the light curve are equal while a negatwe
number would indicate that max I1 is brighter than max I. As
shown in the figure, the ~ 155 day periodicity of the light curve
asymmetry found during 1978-81 was not present during 1985-87.
In the lower portion Fig. 1, the mean light levels from the light
curves are plotted. The mean light level was determined by
averaging the system's brightness at phases 0.0P, 0.25P, 0.50P,
and 0.75P. As shown, the mean light level of VW Cep varies by ~
0.06 mag with time. VW Cep was faintest during 1979/80 and
1986/87 and brightest during 1982-84. The asymmetries appear to
be greatest when the system is the least luminous.
The apparent correlation between the mean brightness of the system
and the size of the light curve asymmetries indicates that starspots
A Decade of UV Astronomy with 1UE, Proc Celebratory Symposium, GSFC, Greenbelt, USA, 12-15 Aprd 1988, ESA SP-281, Vol. 1 (June 1988)
https://ntrs.nasa.gov/search.jsp?R=19890001371 2020-03-20T06:08:12+00:00Z
304 D H BRADSTREET & E F GUINAN
are probably responsible for this behavior. Using the starspot
model developed by Bradstreet (Ref. 3), we were able to reproduce
the observed hght curves with extended starspot regions on the
surface of the larger, cooler star of the system. We were able to
make a preliminary fit to the asymmetric hght curve obtained during
Apnl 1987 shown m Fig. 2. The area of the two spots at this time
was ~ 6% of the total surface area of the larger star, with a
temperature of 450 ° _+150 K cooler than the surrounding
photosphere.
The ~ 155 day cyclic behavior observed during 1978-81 appears to
arise from a small difference between the rotauonal period of the
starspot region and the orbital period of the binary. The 155 day
period is apparently the beat penod between the orbital period and
the rotational period of the photospheric region where the spots
were located at this time. The migration of the spotted region
around the surface of the cooler star produces the varying hght
levels of the maxima and minima of the light curve. At times, when
the spots are small, such as in 1982-84, the asymmetnes are
smallest and the mean brightness of the system is greatest, and
vice-versa as in 1979/80 and in 1986/87. A spot cycle of ~ 7-8
years appears in which the spots reach their maximum areal extent
during 1979/80 and 1986/87. This apparent spot cycle produces
the long-term variation in the mean light level of the binary as
shown in Fig, 1.
3. IUE OBSERVATIONS OF VW CEPItEI
During 1986/87, the light curves of VW Cep were very asymmetric
and the mean light level was about 0.10 mag fainter than observed
dunng 1982-84. Because of this high level of surface activity we
were granted, in April 1987, one US1 discretionary shift to study
the nature and location of the phenomena affecting the light curves.
During one complete orbital cycle, 3 SWP and 4 LWP low
dispersion spectra were obtained. The location of the spotted
region was inferred from ground-based photometry. The SWP
exposures were centered on the orbital phases 0.84P, 0.13P, and
0.60P. These phases were chosen in an attempt to include and then
exclude the suspected active region. The LWP exposures were
centered on phases 0.90P, 0.25P, 0.45P, and 0.75P. Fig. 3
shows a polar view of VW Cep illustrating the exposure intervals
of the IUE spectra. The integrated emission line fluxes of C I1
(M 335) and C IV (M550) are given below the SWP image
numbers (in units of ergs cm -2 S -1 X 10-13). The Mg I1 line fluxes
are given below the LWP image numbers m the same units. The
locations of the starspot regqons obtained from modeling the light
curve are also shown in the figure. Fig. 4 shows the SWP spectra
of VW Cep obtained on 05 April 1987. The strongest emission
features are identified in the figure. The top spectrum (SWP
30713) was taken when the starspots were most visible and
displays the strongest emission features. The other two spectra
were obtained when the spotted regions were only partially visible.
For the first time, phase dependent variations in the TR line
emissions were discovered, most notably C IV, which was 50%
stronger when the spot regions were most visible. From the LWP
observations there appears to be a 20% variation in the level of Mg
II emission, but this occurred during secondary eclipse and, thus,
may be due to the occultation of a localized emitting region in the
chromosphere. Fig. 5 shows the possible phase dependent
variations of the C IV and Mg 1I line emissions, plotted together
with the ground-based light curve obtained during Apnl 1987. As
depicted in the figure, the phases of maximum spot visibility
O'O
oo_
b o¢
p' oo ¢
i°--00_
-00 _
-00_
-OO_
o_
o olo
1978 1979 19_ 1981 1912
,,t ;I l', li[!t_ /_,F\ r_
tJ _/ u
1983 1_SA 1985 1986
I I
//t _ -
/
/
FT'
1987 19nO
, ,,_.' I
VW Cephei
_mmoculolehghilevel
.,=::-
_ _ _"_SPOTMINIMU_-¢_ _ ...o
°o ._ o _
_ .n_ o_ r_ _, o °_-- .... _ _ oog_
SPOTMAX MUM SPOT a4/_XIMUA;
3500 4000 4500 5000 5500 6000 6500 -/'0o(j
JULIANbAY
Figure 1. The long term photometric behavtottr of VW Cep is shown above. The upper portion
represents the changing levels of maximum light m the sense mag(Max 11) - mag(Max 1) where Max
1I and Max I are 0.75P and 0.25P, respectively. The lower panel shows the mean light level of the
system over time.
SURFACE ACTIVITY ON VW CEPHEI 305
tN Ceph 4/26 tt 5/13 1987
Red
i i i i i [ i _ i , ,I _,
.3 0.50 0.00
Phase
Figure 2. Red 0.6600) light curve obtained in April 1987 at
Villanova. The solid curve represents the theoretical light curve
with starspots. The starspots are 450 ° + 150 ° K cooler than the
surrounding photosphere and occupy an area ~ 6% that of the
larger star.
I t I i I [ IVW Cop C IV ,- .
ilCrtt
0 I He II S] II
, S Iv ]
0
1 ,lOl_ 15_3 16!N_ 17'80 1888 1988
_TH (A)
l I I I
VW Cop
8 _OE-1 q\VP 3071 t
400£1,
2 _E-1,
0
IL:t_ 12130 ! ,:g3O 15£'1_
[ [ I I
_)I ;P
LWP 10493
Mg I1 48 9
SWP 30714 0 25P
CIV 509
MglI 410
,,q,,
,,,,' ' "' h,
I" ql]l ' ,_llllllflllrl
o ooi,-1- Hl,,lll!' " -.-o 5o.
LWP 10492 11 "" .... <,i 1] IIll[tull[tl[ /
Mgll 51 2 '",,h,,,,;,,'"
...- /'
\" /SWP 30715
_" CIV 454
SWP 30 _-_ iq ',-,,_ C I1 2 43
C lV 6 8O 0 75P
C II 303
L\VP 10495
gig 11 49 7
Figure 3. Polar view of VW Cep illustxating the exposure
intervals of the SWP spectra. The phase at which the LWP
spectra were taken is also shown. Below the SWP image
numbers is the line flux (in ergs cm 2 s]) x 10 -13 of C 11 and C
IV. The Mg 11 line fluxes are given below the LWP image
numbers.
i
8 _l_l_-14--
6 81_Z-14-
4.00E-14-
1290
I I I
VW Cep
SWP 307 ] 5
[ I I I
z: _% "%
0 60P
Figure 4. SWP spectra of VW Cep obtained 05 April 1987.
The top spectrum (SWP 30713) was centered on 0.84P when
the starspots were most visible. The middle spectrum (SWP
30714) was centered at 0.13P, and the bottom spectrum (SWP
30715) was centered at 0.60P. The insets in the upper
righthand comers show representations of VW Cep at the
appropriate phase of mid-exposure.
306 D H BRADSTREET & E F GUINAN
roughly coincide with the maximum C IV emission. However, the
small number of spectra and their phase distribution are insufficient
to determine unambiguously the association of the UV line
emissions with the stellar regions defined by the starspots. We
plan more observations of VW Cep with IUE during 1988 to test
our results.
4. CONCLUSIONS
VW Cep seems to possess all of the common indications of
enhanced solar activity that are expected from dynamo generated
magnetic fields. The presence of starspots and a possible starspot
cycle are inferred from the ground-based photometry. As this
study shows there are strong chromospheric and transition region
emissions which are indicators of enhanced stellar activity.
Moreover, there appears to be a correlation between the visibility of
the starspots and the strength of the transition region emissions in
the sense expected if the spot forming regions and the transition
region sites were physically related. However, the phase
dependence of the transition region line emissions and the possible
variation of Mg II emission needs to be confirmed with more
observations.
The rapid rotation (Vra = 150 km s-1) of VW Cep and deep
convective layers of its components are in general accord with the
high levels of surface activity |nferred from the ultrawolet and
ground-based observations. However, due to the strong tidal
interactions between the two components, the other assumed
necessary ingredient to most stellar dynamo theories - that of
differential rotation - should be missing or at least substantially
reduced in contact systems like VW Cep. Nonetheless, VW Cep
and other W UMa-type binaries display levels of implied
chromospheric and transition region activity that are among the
greatest of solar-like stars. Perhaps extrapolations of the solar
activity paradigm to these complex binaries are naive, missing some
important mechanism which creates and sustains such large and
long-lived active regions. During 1988 we have been granted time
on the IUE satellite to secure more detailed ultraviolet observations
of VW Cep and these observations should answer some of these
questions (or possibly raise new ones) concerning the study of
stellar activity and dynamo theory at extreme rotational velocities.
We wish to thank Dr. Yoji Kondo for granting us a discretionary
shift with the 1UE at a time when the target was most active. This
work was partially supported under Grant #87-090 from NASA
which is gratefully acknowledged.
5. REFERENCES
Parker E N 1986, in Lecture Notes in Physics, No. 254,
Cool Stars, Stellar Systems, and the Sun; eds. M Zeilik and
D M Gigson; (Springer-Verlag: New York), p. 341.
2. Gilman P A 1980, in IAU Colloq. 51, Stellar Turbulence,
eds. D Gray and J L Linsky; (Springer-Verlag: New York),
p. 19.
3. Bradstreet D H 1985, Ap. J. Suppl. 58, 413.
4. Rucinskl S M & Vdhu O 1983, M. N R. A S, 202, 1221.
5O
40
o
×
30
i
7.C
?
m 6.
o,
× 5.
4. t-
0110 ma9
C %,%
i.
_"
_SPOTS MOST VISIBLE--
a
t
VW CEPREI APRIL 1987
YELLOW LIGHT CURVE
/ '
:-%.. '¢ ."
I --SPOTS MOST VISIBLE--
-- u-- __ -- --
• .
He IT h _ k
/
e-- -- -- •
i
J
J
X
\
C [ v
-\ /
×--- /
X
/
I I
0.5
PHASE
<
0 tlGINAL
0
I
1.0
Figure 5. The yellow light curve of VW Cep obtained during April 1987 is shown in the top panel.
The phases most affected by the starspots are indicated. The Mg II and C IV line fluxes are plotted
versus phase in the lower panel.
PAGE IS
POOR OUAL4TY


Mapping of surface activity on the W UMa-type system VW Cephei

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server